The present invention relates to an extruded discontinuous fiber-reinforced cement matrix composite and method of making same using extrusion techniques.
Cement and concrete are relatively brittle materials with low tensile strength. In attempts to overcome this deficiency of cement and concrete materials, randomly oriented, short discontinuous fibers have been introduced into the cementitious material as a reinforcing material for the cementitious matrix. However, addition of fibers tends to increase the viscosity of the cementitious matrix and to render the material difficult to handle and place. For this reason, in bulk construction using conventional mixing techniques/equipment, only short fibers (e.g. 25 millimeters length) and low fiber volume fractions (e.g. less than 1%) have been used heretofore. For such reinforced cementitious materials, the fibers do not significantly influence the tensile strength of the matrix. Only after the matrix has cracked do the fibers contribute to strength by bridging existing cracks.
Several techniques are used to make commercial fiber-reinforced cement products. The known Hatschek process was initially developed for production of asbestos composites and is now utilized for manufacture of non-asbestos, short discontinuous fiber (e.g. wood fibers and/or polyethylene pulp) reinforced cement composites. In this process, a fiber-cement mixture with excess water is deposited (e.g. roll coated) on a felt band substrate, vacuum dewatered, calendared, and cured to form a fiber reinforced cement matrix in sheet form. However, this method is suitable only for fiber types which retain cement particles during vacuum dewatering. Composites made by the Hatschek process are brittle and only good for sheet.
U.S. Pat. No. 5,108,679 describes manufacture of discontinuous fiber-reinforced cement roofing products using the known roller and slipper process. In this process, the premixed materials including not more than 4 weight % fibers are compressed by passage through rollers and then slipper to obtain flat reinforced sheets to which the process is limited.
Other manufacturing techniques for fiber-reinforced cement products employ continuous fibers rather than short, discontinuous fibers. For example, the known Reticem process produces cement laminate composites with 20 to 30 continuous fiber mesh layers. In particular, each fiber mesh layer is fed from a mesh supply reel, spray coated with cement, covered with the next mesh layer that is then spray coated with cement and so on to form the multi-layered laminate that is compacted, trimmed to length, and cured.
The known pultrusion process produces continuous fiber-reinforced structural shapes with very high fiber volume ratios. In particular, in practice of the pultrusion process, continuous fiber mats are fed from stationary and roving mat creels to a cement slurry bath for coating. Then, the coated mats are formed to shape and cured under pressure.
The Reticem and pultrusion processes described above are disadvantageous in that they require continuous fibers and the processing/equipment technology for incorporating the continuous aligned fibers in the cement matrix are costly. As a result, these processes has been used for the most part in the manufacture of specialty products, such as thin sheets.
U.S. Pat. No. 4,066,723 describes production of laminated cement sheets wherein an unreinforced sheet of concrete is extruded, reinforcing fibers are then distributed onto the surface of the sheet, and these steps are repeated to produce a flat lamination. The process is limited to production of flat laminated products.
It is an object of the present invention to provide a method of making discontinuous fiber reinforced cement matrix composites, as well the reinforced cement composites themselves, using relatively high volume fractions of discontinuous reinforcing fibers by die extrusion to provide improved mechanical properties in that direction.
The present invention provides a method of making a fiber reinforced cement matrix composite having improved strength wherein hydraulic cement, water, water soluble binder for viscosity control, and discontinuous reinforcing fibers are mixed to provide an extrudable mixture having the reinforcing fibers substantially uniformly dispersed therein. The mixture is extruded through an extrusion die orifice having a desired configuration for the composite to provide increased tensile properties of the cured composite as compared to an unreinforced cement matrix or a cast composite of similar composition. The mixture can be extruded to preferentially align the fibers in the extrusion direction to an extent to increase tensile properties as compared to those in the transverse direction of the extruded composite. Although aligned in the direction of extrusion, the reinforcing fibers remain substantially uniformly dispersed throughout the cement matrix of the extruded shape. Alternately, the mixture can be extruded with little or no preferential fiber alignment and yet achieve improved tensile properties in all directions of the composite as compared to a cast composite of similar composition. The extruded shape then is subjected to a curing operation to cure the cement matrix. The discontinuous fiber reinforced cement matrix can be extruded to have the configuration of a flat sheet, pipe, rod, beam, tube, honeycomb, and other structural shapes.
In practicing one embodiment of the invention, the cement preferably comprises a hydraulic cement, such as Type I portland cement. The weight ratio of water to cement preferably is within the general range of 0.2 to 0.4. The discontinuous reinforcing fibers can be selected from the group consisting of polyvinyl alcohol, carbon, steel, polypropylene, cellulose and others and are present from about 4% to about 10% by volume based on dry constituents of the extrudable mixture. The water soluble binder typically is used with a water reducing agent in the extrudable mixture to adjust viscosity of the mixture to the appropriate level for extrusion pursuant to the invention.
In one embodiment of the invention, silica fume is mixed with the cement constituent in formation of the extrudable mixture. The silica fume preferably comprises silica fume powder having a size not exceeding about 1 micron in aqueous slurry. The weight ratio of silica fume powder to cement is up to 0.30.
In a particular embodiment, the present invention provides a method of making a fiber reinforced cement matrix composite having improved tensile strength in the extrusion direction, strain hardening behavior with improved tensile strain (e.g. at least 1% tensile strain) and improved flexural strength in three-point bending. In practicing this embodiment of the invention, hydraulic cement, water, water soluble binder, and discontinuous reinforcing fibers comprising a hydrophilic polymeric material, such as polyvinyl alcohol fibers, are mixed to provide the extrudable mixture, the mixture is extruded to shape with the discontinuous fibers preferentially aligned in the extrusion direction of the extruded shape, and the cement is cured.
The present invention also provides a fiber reinforced cement matrix composite having a die extruded shape and improved tensile properties in all directions of the composite as compared to a cast composite of similar composition.
The present invention also provides a fiber reinforced cement matrix composite having an extruded shape and having substantially improved tensile strength compared to a cast composite of similar composition, strain hardening behavior with substantially improved tensile strain and improved flexural strength in three-point bending.
The present invention provides a fiber reinforced cement matrix composite having an extruded shape and improved tensile properties in the extrusion direction compared to an unreinforced matrix or a cast composite of similar composition. The composite comprises a cured cement matrix and discontinuous reinforcing fibers dispersed in the matrix and preferentially aligned in the extrusion direction of the composite to an extent to increase the relative tensile properties of the cured composite in that direction as compared to the transverse direction by virtue of extrusion of the cementitious mixture.
The objects, advantages and capabilities of the present invention will become more readily apparent with reference to the following detailed description of certain embodiments along with the following drawings.